This invention relates to radial tires and to reinforcements serving to reinforce the carcass of such tires.
Such carcass reinforcements are known to consist of at least one ply, hereinafter referred to as `radial ply` or `radial carcass ply,` prepared in the form of a semifinished product and consisting essentially of an elastomer matrix reinforced with threads. These reinforcing threads, hereinafter referred to as `radial threads,` are arranged at regular intervals, parallel to each other, in a substantially radial direction (substantially perpendicular, that is, to the circumferential direction), that is to say conventionally a direction making an angle of about 90.degree..+-.10.degree. with the tangent to the circumferential direction of the tire.
Each radial ply normally takes the form of a continuous annular element formed, in fabrication of the carcass reinforcement on a drum, by cutting fabric strips or widths, then assembling the two edges of the cut on the drum by a joint, also known as a `weld,` to connect the two edges. According to the rules of the prior art these fabric strips are always cut between two radial threads and parallel to them, so as not to cut them. Hence the weld thus obtained has an orientation that may be called radial, like that of the reinforcing threads.
Two types of joints or welds are currently in use: the so-called `butt` weld, made by butting the two ends (that is, without superposition of the two edges), and the so-called `lap` weld (that is, with superposition, overlapping, of the two edges). In the case of a radial tire carcass for passenger cars or vans, with carcass reinforced by non-metallic threads, in particular textile threads, a lap weld is ordinarily used.
This invention refers specifically to an improvement made in the lap welds of radial plies forming the carcass reinforcements of radial tires.
As the name implies, the technique of lap welding consists in covering one edge of a ply with the other edge, maintaining a certain distance, and forming the bond by applying pressure. After curing of the tire, this type of weld offers great solidity, due to the larger surface area of adhesion available between the overlapping parts.
However, this lap weld has some disadvantages. Because of a double thickness of fabric, the density of radial threads is in effect multiplied by two in the lapped area. By `thread density` is meant, in known manner, the number of threads per unit length of ply, in a direction perpendicular to the axes of the threads. The presence of these `double` threads (double layer of threads) has the result in particular that under the action of the tire inflation pressure, and hence the application of tension to the radial carcass ply, each radial thread present in the weld will undergo about two times less tension and elongation compared to a radial thread located outside of the weld. Furthermore, in known manner, the radial threads, if subject to thermal shrinkage, will show a different contraction upon curing of the tire according to whether they are inside or outside of the weld.
The above phenomena result in a discontinuity of the properties of the carcass ply at the weld, in particular of the rigidity, and consequently a lack of uniformity in the manufactured tire. This discontinuity and lack of uniformity, greatly localized, may be especially damaging to the appearance, the outward aspect, of the tire, since it generally manifests itself by producing a more or less pronounced hollow in the side walls of the inflated tire. As is known, this depression or deformation of the side walls, described also by the term "indentation," is generally more pronounced, the smaller the modulus of elasticity of the reinforcing thread.
The above fault in appearance, well known and especially visible when for example polyester threads are used, is sometimes disadvantageous to the successful marketing of the tire; if this flaw is too pronounced, it may even make the manufactured tire unmarketable, so that it will be destroyed, which is the more regrettable because this flaw in appearance does not affect safety. Besides, it is known that this non-uniformity of the side walls may impair the smooth of running of the tire.
A great many solutions have been proposed heretofore in an attempt to mitigate the foregoing disadvantages. In the vast majority of cases, these solutions consist in modifying the properties of the radial reinforcing threads or their arrangement in the lapped portion in question.
For example, it has been suggested that use be made of radial threads having different properties, for example threads made of materials of smaller modulus, more intensely twisted threads, or having a smaller thermal contraction, a lower strength, diameter and/or titer (see for example Patent Application FR A 2,624,441, Patents EP B 0,205,824, JP 1988/48723); other solutions that have been suggested consist in increasing the spacing between the threads, in removing the `duplicate` threads after the welding operation, or eliminating them prior to the welding operation (see for example patent applications published under Nos. JP 1991/189131, JP 1993/154938, JP 1993/238203, JP 1993/246207). It has also been suggested that the thickness of the weld may be reduced by methods of crushing or homogenizing the lapped area (see for example EP A 0,406,821 or U.S. Pat. No. 5,021,115, EP A 0,498,215, U.S. Pat. No. 5,240,534), or adding another strip of ply or rubber over the weld (see for example EP A 0,407,134 or U.S. Pat. No. 5,437,751).
All of the solutions cited have the major disadvantage of requiring either the preparation of hybrid calendered fabrics, non-homogeneous that is (use, for example, of radial threads of different kinds and/or properties, different thread densities), prior to the manufacturing operations of tire, or additional steps during said manufacture.
Other solutions have been proposed, fewer in number, that consist in selectively cutting some of the `duplicate` radial threads in one or the other lapped edge of the weld. Thus, only the threads of one of the two edges `participate` in tensing the carcass ply. By way of example, U.S. Pat. No. 4,466,473 describes a process of radial carcass reinforcement manufacture that consists in making incisions, in one of the ply edges, perpendicular to the direction of the radial threads, this edge being then covered with the other edge to form the lap weld. Such methods of incision of fabrics by selective cutting of the radial threads of one of the two edges are especially difficult to handle; in the first place, the cutting must be deep enough so that all the `duplicate` threads are cut, but secondly it is imperative, for reasons of safety, that none of the threads outside of the weld be cut. Such conditions necessitate frequent manual interventions or corrections by an operator, on the fabrication drum, and finally these methods are not compatible with the use of automatic fabricating machines, operating at high speed, and not affording the precision essential to execution of such incisions and welds.
Finally, the many solutions heretofore proposed prove costly or difficult to implement industrially, and none of them has been found entirely satisfactory, especially in terms of productivity, to eliminate or even mitigate the problem of side wall deformation of radial tires, in particular, tires reinforced with polyester threads having a low modulus of elasticity.
Thus there is still a need, and a growing need, for improvement in the appearance of the side walls of radial tires.